Hi all :)
I'm here again.
This time I'll provide more details (explain analyze, data-type, and
indexes), hope it will be enough :)
The query that is performing a plan that i do not understand is the
following:
--------------------
select [some fields from all 3 tables]
from
DATA_SEQUENCES
join SUBSCRIPTION on
SUBSCRIPTION.key1 = DATA_SEQUENCES.key1 AND
SUBSCRIPTION.key2 = DATA_SEQUENCES.key2
join people on
people.key1 = SUBSCRIPTION.people_key1 AND
people.key2 = SUBSCRIPTION.people_key2
WHERE DATA_SEQUENCES.import_id = 1351674661
--------------------
This is the explain analyze:
--------------------
Merge Join (cost=2902927.01..2973307.79 rows=790371 width=240) (actual
time=40525.439..40525.439 rows=0 loops=1)
Merge Cond: ((people.key1 = subscription.people_key1) AND (people.key2 =
subscription.people_key2))
-> Sort (cost=2885618.73..2904468.49 rows=7539905 width=240) (actual
time=40525.268..40525.268 rows=1 loops=1)
Sort Key: people.key1, people.key2
Sort Method: external merge Disk: 466528kB
-> Seq Scan on people (cost=0.00..323429.05 rows=7539905
width=240) (actual time=0.029..5193.057 rows=7539469 loops=1)
-> Sort (cost=17308.28..17318.76 rows=4193 width=16) (actual
time=0.167..0.167 rows=0 loops=1)
Sort Key: subscription.people_key1, subscription.people_key2
Sort Method: quicksort Memory: 25kB
-> Nested Loop (cost=0.00..17055.99 rows=4193 width=16) (actual
time=0.154..0.154 rows=0 loops=1)
-> Seq Scan on data_sequences (cost=0.00..150.15 rows=39
width=16) (actual time=0.154..0.154 rows=0 loops=1)
Filter: (import_id = 1351674661)
-> Index Scan using xpksubscription on subscription
(cost=0.00..431.86 rows=108 width=16) (never executed)
Index Cond: ((subscription.key1 = data_sequences.key1)
AND (subscription.key2 = data_sequences.key2))
Total runtime: 40600.815 ms
--------------------
All the key, key2, and relative foreign keys are int4. Import_id is a
bigint.
I'm not reporting the full create table script 'cause people and
subscription both have lots of fields. I know this can be wrong (lots of
field on big table), but this is an environment born something like 20
years ago and not intended from the start for such a big data volume.
I have the following indexes:
on People:
CREATE UNIQUE INDEX people_pkey ON people USING btree (key1, key2)
CREATE INDEX people_pkey_hash_loc ON people USING hash (key1);
CREATE INDEX people_pkey_hash_id ON people USING hash (key2);
on Subscription:
CREATE UNIQUE INDEX subscription_pkey ON subscription USING btree (key1,
key2)
CREATE INDEX subscription_fk_people ON subscription USING btree
(people_key1, people_key2)
on Data_sequences:
create index data_sequences_key on data_sequences USING btree (key1, key2);
create index data_sequences_id on data_sequences USING btree (import_id);
What i don't understand is WHY the seq scan on people, and how can I cast
the import_id to make it use the index on data_sequences (another useless
seq scan).
Mind that when I run this explain analyze there were no records on
data_sequences. So all the time (40 seconds!) is for the useless seq scan
on people. Both people and subscription have lots of records (10-20.000.000
range).
I'm running 8.4 (haven't tested it on 9.2.1 yet, but we planned to upgrade
ASAP cause we have other queries which will benefit from the
index-only-scan new feature).
Thank you in advance,
--
Vincenzo.
